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Old 11-27-2012, 02:28 PM   #1
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Join Date: Apr 2008
Location: fan franfisco
Posts: 12,692
M90 and M30 hybrid options

(crosspost from here)

I'm taking the early L-Jetronic M90 out of my euro 635csi for a refresh of the internals. I've also got a completely torn-apart M30B35 and a complete B35 harness from an E34 535i in the shop, and have been wondering about clever ways to combine the two to make an engine that is more powerful than either. I don't want to use a B35 crank, though, as the one I have is damaged (the reason the B35 is apart)

From some reading and research I've come up with three alternatives that I'm now deciding between:

B35 cam, front cover and intake manifold, Motronic 1.3 electronics conversion
This would let me keep the M90 head and not lose compression, which is nice, but I don't get the advantages of the B35 head's bigger valves and better port shapes (I'll be using the B35's exhaust manifolds either way, but M90 head + B35 intake manifold = port mismatch)

complete B35 head on M90 block, Motronic 1.3
The main problem with this approach is lost compression, because the B35 head has bigger combustion chambers than the M90 head, as it's designed for domed pistons (B35) rather than flat (M90). However, I get the head's other advantages, including Motronic 1.3, bigger intake valves, better port matching and flow. It is possible to skim the head to bump the compression back up, but that retards the cam timing and needs an adjustable cam gear to fix.

M90 cam and front cover in B35 head, keep everything L-Jetronic
Some people here have mentioned that L-Jet is way more responsive and revvy than Motronic 1.3. I am not sure if that's true or not, but this option is attractive if it's concluded that the conversion to Motronic is not beneficial. I lose compression, but gain valve size and flow. This is not really a great option.

Any thoughts on any of these alternatives are appreciated. Which would you do?
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